In recent years, there have been proposed and developed various impeller manufacturing technologies in which a shroud and each impeller blade are welded or joined together by heating (e.g., ultrasonic-welding) their mating faces extending radially outward from the opening of the shroud. One such impeller manufacturing technology has been disclosed in Japanese Unexamined Patent Application Publication No. 2011-122457 (hereinafter is referred to as “JP2011-122457”).